Introduction

While quenched steel with carbon above 0,6%, the temperature of the end martensite transformation is below zero, thus the transformation of austenite into martensite is incomplete and this remaining cooled austenite is called residual austenite.
Residual austenite is a steel structure which during cooling at martensite transformation temperature is not completely converted into martensite and remains unchanged at room temperature together with martensite.
The structure of residual austenite is metastable, during exploitation it may partially transform into bainite, whereas during quenching this transformation may be caused by the freezing out processing. The transformation of residual austenite into bainite is connected with volume change, whereas diminishing the content of austenite in martensite by 1% causes a 0,07% increase of its volume.
With regard to use residual austenite indicates defects in form of low hardness, but with regard to physical features it is characterized by low, with regard to martensite electrical conductivity and magnetic penetrability. The differences in electrical conductivity and magnetic penetrability of austenite with regard to magnesite constitute the basis used for detecting residual austenite with the eddy current method. Detection and evaluation of the of the amount of residual austenite is possible using the roentgen diffraction method, mocroscopic observation in microsections, microhardness measurements (in this case detection of its significant amount - above 10%).
The eddy current method is used for the quantitative evaluation of residual austenite contents in the martensite structure.

Physical principles of the method

The principle physical phenomenon of applying the eddy current method for evaluating the amount of residual austenite in the structure of quenched steel is magnetic induction, involving the influence of the changeable magnetic field on the studied area, found under the probe. In effect of such activity eddy currents in the studied area are induced, producing own magnetic fields; which following Lenz's rule are directed adversely to the induction field this decreasing its intensity (Fig.1)

Ho - field produced by the probe,
Hp - field ensuring the measuring signal,
Hw - field produced by eddy currents.(attenuated because of low AS conductivity).

Fig1: A diagram of the principle of the contact probe during the study of the metal material structure:

Fig 2: A diagram of magnetic field whlie testing the structure of the material containing residual austenite (AS):

The intensity of the magnetic field produced by eddy current is depended on electrical conductivity and magnetic permeability of the studied area. In case of a uniform structure, when the conductivity of the material is high, the intensity of the induced magnetic field is big and signal received by probe Hp is small.
Structure defects decrease conductivity of the studied material, and then the intensity of the induced magnetic field is small and the signal received by the probe Hp is big (Fig.2). Low conductivity of austenite is a defects of the structure in case of residual austenite in the martensite structure, which with regard to the magnesite structure is as 1:5. Eddy currents produced in the studied area are subject to excitation in effect of small conductivity of austenite grains in the structure of the studied material.

Research equipment

Eddy current devices for detecting residual austenite comprise:

variable current generator with frequency controlled from average (KiloHertz) to high (MegaHertz),

contact probe with constant location with regard to the studied, or material or external passage with appropriately converged field, with regard to the elimination of the influence of border conditions, which occur in case of controlling the object of finite dimensions,

amplifier with controlled amplification with regard to the adjustment of the indication scope (output signals) to the control system and part segregation,

decoder preparing the signal of measuring results to information and automatics indicators

The detection of residual austenite in fact requires average frequency, however for comparison reasons (reference) with a different recognized method, it is recommended to use high frequency, as with high frequency of eddy currents the penetration depth is comparable in the diffraction method and eddy current method.
Attention should be given in the fact, that penetration of eddy currents in residual austenite will be slightly deeper than in the martensite structure of steel, as austenite shows low electrical conductivity. The signal originatimg from the austenite structure will be amplified in effect of the influence of the structure found at greater depth. There will be no error as the method of measurement is compartable and the samples made for reference purposes will have the same structure as the studied part.
The application of high currents in case of bearing steel, has also a different practical aspect, the small penetration of eddy currents delivers a concentrated measuring signal from the surface of studied element, and the grinding burning occurs in the surface layer.
The indicated eddy current devices referring to the burning of the surface have an opposite sign to indications originating from the structure of residual austenite. During one control process grinding burning can be detected as well as residual austenite, with reference made to samples for determining limits of values allowed for residual austenite and the state of the surface layer after grinding.
For detecting and percentage evaluation of the participation of the amount of austenite in the quenched structure of hyper-eutectoidal steel, devices manufactured by IMP type WIROTEST 202 and WIROTEST 12 finish (Table 1.) are applied. These devices allow to detect and evaluate the content of residual austenite as well as form the signal for part segregation with austenite content above the allowed amount, as well as parts with grinding burning

Calibration of devices for residual austenite control in elements after thermal processing

The determining of sorting limits of steel parts after thermal processing in order to eliminate these, which indicate exceeded allowed content of residual austenite, requires elements of identical shape and dimensions, as the studied parts, and with known content of residual austenite. Such elements serve to define the sorting thresold, during manual control as well as automatic.
It is known, the residual austenite is not a stable structure and after some time is transformed into a bainite structure, so elements used for calibrating sorting thresholds will be unstable, and thus unrealiable. Thus special reference samples showing structure stability should be used.

Fig. 3. A diagram of the sample with residual austenite for calibrating eddy current devices

Thus they have been replaced with elements with a martensite structure mixture fully
transformed by the zero processing and mounted in openings with inserts of austenite steel (Fig.3). Thus prepared elements for calibration will be stable with time and will not cause any indication changes during exploitation.
The percentage share of the inserts made austenite steel in the martensite structure is refered to the visual field of the probe in the given location. Every probe after performance is given a characteristics, in which the visual field is given, determined using special devices defining the visual field at different distances from the tested object.

Results of tests of the occurrence of residual austenite in bearing steel

Bearing elements made of bearing steel were subject to studies. External ring-type elements with an internal race of 20 mm diameter and 12 mm height (Fig.4) were subject to tests using the contact probe and internal volumetric ones. Results of tests are given in Table 2.

Fig 4. Graph of the dependence of Wirotest 202 indications as function of the residual austenite content

No

% content of residual austenite

Wirotest 202 indications

1

0

50

2

5

25

3

8

12

4

10

0

5

11

-3

6

11

-4

7

12

-8

8

14

-20

9

16

-25

10

20

-35

Table 2. Results of tests of residual austenite in bearing rings

Segregation of bearings, with regard to residual austenite was performed with the aid of WIROTEST 202 and WIROTEST 12 finish. Selected rings with defined indications were subject to metalographic tests, in order to state whether residual austenite occurs, and then using the diffraction method, the percentage content of residual austenite.
The showm in Fig. 4 graph of the dependence of the WIROTEST 202 as a function of the residual austenite content, allows to evaluate the content of residual austenite in steel in the scope for 5 +/-100%.
It should be emphasised, that further tests will allow to obtain a higher precision of detectability of residual austenite, especially within the scope below 5%.

Conclusions

The eddy current method may be used for controlling the share of residual austenite in the structure of quenched hyper-eutecloidal steel in the martensite.
The application of WIROTEST 202 device with high frequency magnetic currents, will allow for simultaneous detection of parts with burns, which can occur during grinding.

The elaborated method of applying elements for calibration of devices composed of the body with a nartensite structure, with elements made of austenite steel placed in it, allows to ensure calibration repeatability.

Simple service of control devices, easy automation of the control process allows for extensive application in part manufacturing processes.